This invention relates to a process for the preparation of 4-bromophthalic anhydride by reacting 4-chloro tetrahydrophthalic anhydride with bromine in the presence of iron. 4-Bromophthalic anhydride is useful as an intermediate for the preparation of various commercial products including polymers, dyes and plasticizers. It is particularly useful in the preparation of dianhydride monomers such as oxydiphthalic dianhydride which may be polymerized with a suitable diamine to form a polyimide. 4-Bromophthalic anhydride has been prepared by the reaction of phthalic anhydride, in aqueous alkali, with bromine, and subsequent acidification and dehydration (E. T. Sabourin, et al., J. Org. Chem., Vol. 48, 5137 (1983)). Our process produces 4-bromophthalic anhydride in better yields than we have been able to achieve using the method reported by Sabourin, et al.
4-Chloro tetrahydrophthalic anhydride may be prepared by the condensation of 2-chloro-1,3-butadiene with maleic anhydride. This reaction produces mostly 4-chlorotetrahydrophthalic anhydride with the double bond in the 4 position. A small percentage of product has the double bond in the 3 position. The presence of two isomers has no apparent effect upon the reactions which are the subject of this invention.
The dehydrogenation of tetrahydrophthalic anhydrides to yield phthalic anhydrides has been observed. For example, Bergmann, J. Amer. Chem. Soc. 64, 176 (1942) discloses the aromatization of tetrahydrophthalic anhydride products of Diels-Alder reactions. The author discloses that dehydrogenation occurred when the tetrahydrophthalic anhydride product is boiled in nitrobenzene. However, it is further disclosed that dehydrogenation does not occur when p-bromonitrobenzene, p-chloronitrobenzene, or m-dinitrobenzene in xylene is employed.
U.S. Pat. No. 4,560,772 to Telschow discloses the reaction of 4-methyltetrahydrophthalic anhydride with excess sulfur and a catalytic amount of zinc oxide and 2-mercaptobenzothiazole to produce 4-methylphthalic anhydride and hydrogen sulfide.
U.S. Pat. No. 4,560,773 to Telschow discloses a similar reaction between the electron rich 4-methyltetrahydrophthalic anhydride and bromine in the presence of a catalytic amount of an acid acceptor such as dimethylformamide or pyridine in the liquid phase.
Skvarchenko et al., Obshchei Khimii, Vol. 30, No. 11. pp. 3535-3541 disclose the aromatization of chloro-substituted tetrahydrophthalic anhydride by heating with phosphorus pentoxide. In the aromatization process described, however, decarboxylation also occurs with the formation of the corresponding chloro-substituted benzene compound. The preparation of various other tetrahydrophthalic acids and anhydrides and various methods for dehydrogenation and aromatization thereof are reviewed by Skvarchenko in Russian Chemical Review, No. 1963, pp. 571-589.
A co-pending application, Ser. No. 07/405,606, discloses that 4-chlorotetrahydrophthalic anhydride reacts with elemental bromine to form 4-chlorophthalic anhydride. 4-Bromophthalic anhydride is sometimes found as a side product in low percentage, but is not economically recoverable.
Iron catalyzes the bromination of aromatic molecules (ORGANIC CHEMISTRY, Louis F. Fieser and Mary Fieser, 3rd Edition, D. C. Heath and Company, Boston 1956, pages 644-645 and FUNDAMENTALS OF ORGANIC CHEMISTRY, 2nd Edition, T. W. Graham Solomons, John Wiley & Sons, New York 1986, pages 445-446 and 455). For example, benzene reacts with bromine, in the presence of iron, to form bromobenzene. Chlorobenzene, under similar conditions, forms bromochlorobenzene. Halogens and carboxyl groups, when substituted on a benzene ring, tend to de-activate the ring to further substitution. (Solomons, p. 455). The presence of three de-activating groups on the ring would be expected to cause ring bromination to be slow. In fact, we have found that under the reaction conditions used for the aromatization of 4-chlorotetrahydrophthalic anhydride, 4-chlorophthalic anhydride does not react with bromine in the presence of iron.